We are proposing to study the structure of an intermediate that is generated during the assembly of the essential tyrosyl radical/diferric cluster cofactor of the R2 subunit of ribonucleotide reductase from E coli. Stubbe and coworkers have demonstrated that this kinetically competent intermediate (called X) can generate the tyrosyl radical using rapid freeze quench (RFQ) Mvssbauer and EPR spectroscopies in conjunction with stopped flow absorption spectroscopy. Since RFQ samples can be made that consist of 60-80% X, we propose to use RFQ EXAFS to distinguish between these two structural motifs. This structural information will provide insight into the mechanism of the assembly process and broaden our understanding of oxygen activation by dinuclear iron proteins.